Davis. — Si(jii ificnnt Ftafurex of l\nf-li(>i<h red ('odKfs. 13 



The absence, of talus at the base of sucli bluffs shows that they are 

 still washed by storm waves, but the strength of attack is not great. 

 Indeed, it is probable enough that the greater part of the abrasion that 

 these low spur-end clifTs attest was accomplished by waves that rolled in, 

 little impeded, from the open ocean, shortly after the last period of 

 submergence and before the present barrier reef was built up to sea-level. 

 This implies that the submergence was rapidly accomplished and was 

 followed bv a pause. The form of the abraded rock platform on many 

 spur-ends supports that view ; for its outer border is about in line with 

 the sloping ridge-crest above the cut-off bluff, as it should be if all the 

 abrasion had been accomplished since submergence took place. If some 

 abrasion had been accomplished during the progress of slow submergence 

 the outer border of the platform could not be so nearly in line with the 

 ridge-crest. Detailed observation of spur-end bluffs and platforms is 

 therefore desirable. 



If the above suggestion as to the origin of the spar-end bluffs be 

 correct, their height will not be so good an indication of the time since 

 siibmergence as may be found in the breadth of the fringing and barrier 

 reefs, or in the area of the bay-head deltas. If the reefs are narrow and 

 the deltas are small (due consideration being given the drainage-area and 

 slope of their streams) the latest submergence must be recent ; if the 

 reefs are broad and the deltas are large enough to fill the embayments 

 the submergence must be less recent, though by no means of ancient date. 

 Careful record of all these features should be made. 



Barrier Eeefs. 



The transfoi-'uation of original discontinuous fringing reefs on the spur- 

 ends of a slightly submerged coast into a nearly continuous barrier reef 

 a mile or more outside of a more deeply submerged coast involves the 

 circupiforoiitial extension of the fringing reefs as they grow upward, so 

 'hat they shall close most of the broad breaches that would otherwise 

 mark the sites of the original embayments. The barrier reef on the eastern 

 side of Tahiti is a good example of discontinuous growth still interrupted 

 by wide passages ; on the w^estern side the reef is more continuous. It 

 may be noted in this connection that barrier reefs in Fiji are, unlike those 

 of Tahiti, more generally interrupted on their leev/ard than on their wind- 

 ward side : this may be due to the injury to coral-growth caused by the 

 drift of fine sediment in the lagoon-waters. 



The depth of a barrier-reef foundation, or the amount of submergence 

 that has taken place since the barrier reef began its growth as a fringing 

 reef, cannot be determined readily, because it is impossible to say whether 

 the upgrowth of the reef has taken place vertically or on an inward or an 

 outward slant. The rate of submergence appears to be the chief factor 

 in determining the angle of upgrowth (Davis, 1916c). The vertical depth 

 from a barrier reef to the underlying rock can be more safely estimated, 

 because the submarine slope of the foundation mass can often be fairlv 

 well inferred. 



It is not necessary here to proceed merely on the empirical principle, 

 introduced by Dampier and followed by Darwin, to the effect that 

 " a considerable degree of relation subsists between the inclination of 

 that part of the land which is beneath the water and that above it," for 

 it is now possible to infer the declivity of a submarine slope more 

 reasonably by observation of the structure and form of the land-mass 



